This invention relates to a novel method of blackening surfaces of steel parts, such as aperture masks and support frames that are used in color television picture tubes. The method employs a wet nitrogen processing atmosphere which is not injurious, obnoxious or flammable, and produces deep black layers which are both adherent to, and protective of, the steel parts.
The surfaces of low-carbon steel parts, such as the aperture masks and support frames that are used in color television picture tubes, are blackened for one or more reasons. Among these reasons are: to provide resistance to rusting, to increase infrared emissivity and to increase absorption of visible light.
For uses in television picture tubes, the surfaces of the parts should be a deep black, not blue or brown or gray, in order to realize optimum spectral-light absorption and high infrared emissivity. The blackening should not flake off or crumble when the part is flexed or when it is thermally cycled. The blackening should be chemically stable in air up to about 475.degree. C. and should provide chemical protection for the part in humid atmospheres. The blackening should be produced without distorting the part and without increasing the magnetic coercivity of the part, all at relatively low cost per part.
Although prior methods of blackening are effective, there is always a need to reduce the unit cost, to reduce hazards to health in the factory and to improve the quality of the product. Prior methods include steam blackening, exothermic blackening and dry-nitrogen blackening.
In steam blackening, the parts are heated to about 450.degree. to 520.degree. C. in a protective atmosphere, and then are exposed to superheated steam. This method is practiced in batch furnaces and is not economically practiced in continuously-operating furnaces. The method produces an adherent coating which has a distinct bluish cast.
In exothermic blackening, the parts are heated to about 550.degree. to 650.degree. C. and then exposed to an atmosphere of burnt fuel gas, which is essentially a gaseous mixture of CO, CO.sub.2, H.sub.2, H.sub.2 O, N.sub.2 and O.sub.2. The atmosphere may be flammable and, if uncontrolled, may be both injurious and obnoxious to factory personnel. The method requires many precise process controls to maintain suitable conditions for blackening.
In dry-nitrogen blackening, the parts are heated to about 650.degree. C. in a protective atmosphere, and then exposed to a dry-nitrogen atmosphere. Dry nitrogen ordinarily has a dew point of less than -30.degree. C. The atmosphere used in this method is safe and odorless, and the required process controls are relatively simple. However, the method produces a dull, soft-gray-appearing coating which has less than desirable physical properties.